1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a light transmissive type ferroelectric liquid crystal display device having a matrix type display panel.
2. Description of Prior Art
FIGS. 7 and 8 show fundamental structures of a conventional matrix type display with the use of a light transmissive type ferroelectric liquid crystal display.
In FIGS. 7 and 8, polarizers 1 and 9 arranged to have crossed Nicols relation, are formed oppositely to each other, and a light source (not shown) is location on the outer surface of the polarizer 9. Transparent substrates 2 and 8, made of glass, are arranged on the inner surfaces of the polarizers 1 and 9, respectively. Sixteen transparent conductive scanning electrodes 4 (L.sub.1 to L.sub.16) having a strip shape are formed in parallel with each other on the inner surface of the transparent substrate 2, and sixteen transparent conductive signal electrodes 7 for applying a signal having a strip shape are formed in parallel with each other on the inner surface of the transparent substrate 8. A ferroelectric liquid crystal layer 5 is formed between the transparent substrates 2 and 8 and is sealed by a sealing element 6. Respective drivers 3 are connected to the respective scanning electrodes 4 (L.sub.1 to L.sub.16) so as to apply a voltage Vc thereto. Respective drivers 10 are connected to the respective signal electrodes 7 so as to apply a voltage Vs thereto.
The operation of the conventional matrix type display panel will be described below.
The portions where the respective electrodes 4 and electrodes 7 are overlapped become picture elements, respectively. In order to bring one of the picture elements into a bright state, it is necessary to apply the voltages Vc and Vs to the corresponding electrodes 4 and 7 so as to satisfy the following equation (1) (or (2)) for .tau..sub.0 seconds or more. EQU Vc-Vs.gtoreq.Vth.sub.1 ( 1) EQU Vc-Vs.gtoreq.-Vth.sub.2 ( 2)
wherein Vth.sub.1 and Vth.sub.2 are threshold voltages of the liquid crystal and .tau..sub.0 is a real number larger than zero.
On the other hand, in order to bring one of the picture elements into a dark state, it is necessary to keep another equation (2) (or (1)) for .tau..sub.0 seconds or more. Normally, when a direct current voltage is applied to the ferroelectric liquid crystal layer 5, the characteristics of the equations (1) and (2) tend to be shifted. In order to avoid this, before applying a positive voltage V.sub.0 (volts)=Vc-Vs (V.sub.0 .gtoreq.Vth.sub.1) to the liquid crystal layer 5 for .tau..sub.0 seconds, a negative voltage -V.sub.0 (volts)=Vc-Vs (-V.sub.0 .ltoreq.-Vth.sub.2) must be applied thereto for .tau..sub.0 seconds. On the other hand, before applying the negative voltage -V.sub.0 (volts)=Vc -Vs to the liquid crystal layer 5 for .tau..sub.0 seconds, the positive voltage V.sub.0 =Vc-Vs must be applied thereto for .tau..sub.0 seconds. That is, it takes at least 2.tau..sub.0 seconds to rewrite the picture elements defined on one scanning electrode 4. The aforementioned switching operation between the positive and negative voltages is referred as to the selection of the scanning electrode 4 hereinafter, and the time interval necessary for the aforementioned switching operation is referred to the time interval for selecting the scanning electrode 4. It is to be noted that the voltages Vth.sub.1, -Vth.sub.2 and .tau..sub.0 are determin the ferroelectric liquid crystal to be used.
In the case that images are displayed on the matrix type display panel shown in FIG. 7, on the assumption that the time for selecting one electrode 4 is 2.tau..sub.0, it takes selecting the sixteen scanning electrodes L.sub.1 to L.sub.16 sequentially. A time period needed for selecting all of the sixteen scanning electrodes L.sub.1 to L.sub.16 sequentially is referred to the frame period T.sub.F hereinafter.
In the case of the matrix type display panel shown in FIG. 7, there are the sixteen scanning electrodes 4; However, in the case that there are M scanning electrodes 4 in a matrix type display panel, the frame period T.sub.F is expressed as follows: EQU R.sub.f M.times.2.tau..sub.0 ( 3)
wherein M is a positive integer.
When a ferroelectric liquid crystal having the time interval .tau..sub.0 =100 micro seconds is used and the number M of the scanning electrodes 4 is 400, the frame period T.sub.F is 0.08 seconds.
In order to hold one of picture elements in a dark state, it is necessary to apply the negative voltage thereto. However, when the scanning electrode 4 corresponding thereto is selected again after the frame period T.sub.F, it becomes necessary to apply the positive voltage thereto for the time interval .tau..sub.0. Due to this, the picture element is brought into a bright state temporarily. Namely, the picture element becomes the bright state every frame period T.sub.F (=0.08 seconds).
Thus, when the bright and dark states of the picture element are switched in a period larger than 1/60 seconds, a flicker is caused on the display panel affecting a human's eyes resulting in the display panel producing unsightly images.
In order to overcome the aforementioned disadvantages, a matrix type ferroelectric liquid crystal display device further comprising a back light source 11 as shown in FIG. 9 in addition to the structure shown in FIG. 8 is proposed. The back light source 11 is arranged on the outer surface of the polarizer 9, and the light intensity thereof is controlled by a control circuit 12 periodically as shown in (a) of FIG. 10. While the light intensity of the back light source 11 is kept relatively low, the control circuit 12 controls the drivers 3 and 10 so as to select individual scanning electrodes 4 sequentially.
In the aforementioned conventional display device comprising the back light source 11 and the control circuit 12, when at least one picture element is rewritten, the back light source 11 is turned off so that the whole matrix display panel becomes relatively dark. On the other hand, except for the respective time intervals for rewriting the picture elements, the back light source 11 is turned on.
The aforementioned conventional display device shown in FIG. 9 can prevent the display from flickering. However, since the scanning electrode 4 cannot be selected for the time interval when the light intensity of the back light source 11 is kept relatively high, the frame period T.sub.F increases. If the ratio of the time interval for the high light intensity to the time interval for the low light intensity of the back light source 11 is 1:1, the frame period T.sub.F becomes twice as long as that in the aforementioned display device not having the back light source. In the case that the time interval .tau..sub.0 is 100 micro seconds and the number M of the scanning electrodes 4 is 400 as described above, the frame period T.sub.F becomes 0.16 seconds, resulting in that the movement of the images becomes slow in a moving image display.